In reply to alexm198:
ideally have a whole arsenal to choose from.
for nasty crazed stuff where a puncture is a game-ender then i take a thermarest with either a slab of z-lite or a sheet of 2mm astro foam with a silver facing if its warmer than about -5c. this system packs tighter and still handles if punctured and combined with a pack and the rope. ropes, when coiled large and laid under other insulation actually insulate really well (on sleepless nights ive contemplated a special bag it would go in to make an intentional system...).
for valley-based stuff or where its just a night or two and/or the chance of puncture is minimal (ie not on cramped ledges etc) then an exped down mat is unbeatable. straight onto snow at -22c is taken for granted, tho a ground sheet or tent floor helps with moisture. only issue is they dont fit inside bivvy bags well. the x-therm does the same job with more noise.
a while back radson mentioned the nemo zor in a similar post. ive got 2 3/4 sized ones that combine well for serious cold, or alone to about -3c. they fold no smaller than an exped tho. for real cold they are a good option as still smaller than a foamy and hopefully less chance of puncturing 2 (...?).
if you do stick with the old school single-big-foamy; rolled up on your pack is a pain in the ass. modify it to fold concertina-style, or roll loose and line your pack with it.
a good system ive seen in japan is a z-lite (made of dimply foam) inside an astro foam sleeve.
another option still is a klymit mat (skeleton of an inflatable) inside the astro foam sleeve.
id keep an eye on that company as i dont doubt sooner or later a down filled version will arise (they are currrently messing about with argon inflation...)